In the mathematical field of order theory an order isomorphism is a special kind of monotone function that constitutes a suitable notion of isomorphism for partially ordered sets. Whenever two partially ordered sets are order isomorphic, they can be considered to be "essentially the same" in the sense that one of the orders can be obtained from the other just by renaming of elements. Two strictly weaker notions that relate to order isomorphisms are order embeddings and Galois connections.

Formally, given two partially ordered sets (S, ≤) and (T, <=) an order isomorphism from (S, ≤) to (T, <=) is a surjective function h : S → T such that for all u and v in S,

h(u) <= h(v) iff u ≤ v.

In this case, the posets S and T are said to be order isomorphic. Note that the above definition characterizes order isomorphisms as surjective order embeddings. It should also be remarked that order isomorphisms are necessarily injective. Hence, yet another characterization of order isomorphisms is possible: they are exactly those monotone bijections that have a monotone inverse.

An order isomorphism from (S, ≤) to itself is called an order automorphism.